A flame-retardant resin composed of a low molecular weight styrene series resin and a low polymerization degree vinyl chloride series resin is an excellent material in view of its mechanical properties and cost. Recently the use of the resin as housing materials for business machines and instruments has been increased. However, runner-less molding has been widely used as one of useful means from the environmental problem of plastics, etc.
However, the foregoing material has difficulties that black stripes and flash are liable to form on the molded products thereof and a molding work through hot runner is reluctant to apply. The reason is believed to be that since the material contains, in particular, a vinyl chloride series resin, the melt viscosity of the resin at injection molding is high and the fluidity thereof is low, which causes residence burning at the hot runner portion, increased shearing heating of a metal such as a screw, a cylinder, a mold, etc., and thermal decomposition of the resin, thereby causing black stripes, flash, etc., on the molded product.
For overcoming the above-described difficulties, it has generally been practiced to add a large amount of a stabilizer or a lubricant to the resin. However, these methods sometimes give large problems that the additive added during molding attach to a mold to lower productivity, and the appearance of the molded products becomes inferior, etc.